Field of the Disclosure
The present disclosure relates to a wavelength-division multiplexer and de-multiplexer, and more particularly to a wavelength-division multiplexer and de-multiplexer with reduced deformation due to temperature changes.
Brief Description of the Related Art
Wavelength-division multiplexing (WDM) technique is widely used in fiber-optic systems for increasing the system's bandwidth. Conventional WDM modules include thin-film filters (TFF) or optical combiner, arrayed-waveguide gratings (AWG) and/or fiber-bragg gratings (FBG). The thin-film filter or optical combiner is advantageous due to its flat passband, low crosstalk and low temperature-dependence.
The thin-film filter or splitter employed for a WDM module or wavelength-division de-multiplexing (WDDM) module includes multiple layers of dielectric films. These layers of dielectric films form a multi-cavity Fabry-Perot interferometer. In such an interferometer, a center wavelength of its passband is dependent on the angle of incidence as indicated below:
  w  =            w      0        ⁢                  1        -                                            (                              1                                  n                  e                                            )                        2                    ⁢                      sin            2                    ⁢          θ                    
where w is the center wavelength of the filter or splitter with an external incident angle of θ; w0 is the center wavelength of the filter or splitter with normal incidence; and ne is an effective refractive index of a cavity spacer of the filter or splitter. From the above equation, the filter or splitter may have a passband with a shorted wavelength due to an increased angle of incidence on the filter or splitter. This filter or splitter has the above property so as to be tuned to meet wavelengths required by ITU (International Telecommunication Union).
FIG. 1 illustrates light paths of a conventional 8-channel WDDM module. Referring to FIG. 1, the WDM module 1 includes multiple filters or splitters 10 each configured to split a multiplexed light beam into an output light beam to pass through said each of the filters or splitters 10 to form a channel coupled with a fiber and another multiplexed light beam to be reflected from said each of the filters or splitters 10 to another one of the filters or splitters 10. The WDM module 1 includes another filter or splitter 11 configured to process a multiplexed light beam into an output light beam to pass through the filter or splitter 11 to form a channel coupled with a fiber. Thereby, the WDDM module may de-multiplex an input light beam at its input into the output light beams with different wavelengths λ1-λ8 respectively at its respective outputs coupled externally to the respective fibers.
Alternatively, the same configuration can be used for a WDM module by reversing the light propagation directions. FIG. 2 illustrates light paths of a conventional 8-channel WDM module. Referring to FIG. 2, each of the filters 10 acting as optical combiners may receive an input light beam, i.e. a channel of the WDM module, from one of the fibers to pass through said each of the filters or combiners 10 to be combined with a multiplexed light beam from another one of the filters or combiners 10 into another multiplexed light beam to propagate to the other one of the filters or combiners 10. The filter 11 may receive an input light beam, i.e. a channel of the WDM module, from one of the fibers to pass through the filter 11 into a multiplexed light beam to propagate to one of the filters or combiners 10. Thereby, the WDM module may multiplex the input light beams with different wavelengths λ1-λ8 at its inputs coupled externally to the respective fibers into an output light beam at its output.